a All operations other than those operating under 14 CFR 121 and
14 CFR 135. Data for 1996 are estimated using new information on nonrespondents
and are not comparable to earlier years. Mileage in source is multiplied by
1.151 to convert to nautical-miles for 1985-1997.

b U.S. Department of Transportation, Federal Highway Administration
(FHWA), provides data separately for passenger car and motorcycle in its annual
Highway Statistics series. However, the 1995 summary report provides
updated data for passenger car and motorcycle combined. Passenger car figures
in this table were computed by U.S. Department of Transportation, Bureau of
Transportation Statistics, by subtracting the most current motorcycle figures
from the aggregate passenger car and motorcycle figures.

c In July 1997, the FHWA published revised vehicle-miles data for
the highway modes for many years. The major change reflected the reassignment
of some vehicles from the passenger car category to the other 2-axle 4-tire
vehicle category. This category was calculated prior to rounding.

d Motor bus and demand responsive figures are also included in the
bus figure for highway.

e Prior to 1985, excludes demand responsive and most rural and smaller
systems funded via Sections 18 and 16(b)2, Federal Transit Act. The series is
not continuous between 1980 and 1985. Transit rail modes are measured in car-miles.
Car-miles measure individual vehicle-miles in a train. A 10-car train traveling
1 mile would equal 1 train-mile and 10 car-miles.

f Amtrak began operations in 1971.

g Although both train-miles and car-miles are shown for rail, only
train-miles are included in the total. A train-mile is the movement of a train,
which can consist of multiple vehicles (cars), the distance of 1 mile. This
differs from a vehicle-mile, which is the movement of 1 vehicle the distance
of 1 mile. A 10-vehicle train traveling 1 mile would be measured as 1 train-mile
and 10 vehicle-miles. Caution should be used when comparing train-miles with
vehicle miles.

h 1960-65, motorcycle data are included in passenger car, and other
2-axle 4-tire vehicle data included in single-unit 2-axle 6-tire or more truck.